Reflex oscillator



Sept. 4, 1951 W. G. SHEPHERD REFLEX OSCILLATOR Filed Nov. 27, 1948 FIG.

0 O 4 W4 /m 1 Q 0 f Q s f, m. H m v, 3 w A m E G u mm F R 3F A w 6 3 m m m m w ATTOPNEV Patented Sept. 4, 1951 REFLEX (PSCILLATOR William G. Shepherd, St. Paul, Minn., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 27, 1948, Serial No. 62,395

2 Claims.

This invention relates to electron discharge devices and more particularly to tunable reflex oscillators.

Such oscillators, of which a typical construction is disclosed in Patent 2,411,913, granted December 3, 1946, to John R. Pierce and William G. Shepherd, comprise, in general, a high frequency circuit element, such as a cavity resonator, having a gap therein, an electron gun opposite one end of the gap, and a repeller electrode opposite the other end of the gap. The directcurrent electron stream projected by the electron gun ls velocity varied in its passage across the gap and the velocity varied stream is reversed in the region between the gap and the repeller electrode and directed into the gap to contribute ener y to the field within the high frequency circuit element, thereby to sustain oscillations. The frequency of oscillation may be varied by tuning the circuit element. This may be efiected expeditiously in the case where the circuit element is a cavity resonator by flexing a wall of the resonator, specifically by flexing the wall of the resonator opposite the repeller electrode.

The operating characteristics of such a device are dependent upon the transit angle of the electrons in the region between the gap and the repeller electrode, both forward and reverse motion of the electrons being considered. For optimum operation, the transit angle should be substantially equal to (rt-V cycles of the operating frequency, n being an integer. The transit angle is dependent upon the distance between the gap and the repeller electrode and the potential of this electrode. If the resonator is tuned by flexing a wall thereof as described hereinabove, the gap to repeller electrode distance changes. He retofore, this has necessitated changing the repeller voltage to obtain optimum operation, specifically maximum output power.

One object of this invention is to simplify the operation of tunable reflex oscillators. More specifically, one object of this invention is to enable attainment of substantially optimum operation of such devices over a wide range of frequencies at a fixed repeller electrode voltage.

In accordance with one feature of this invention, in a tunable reflex oscillator of the general construction described hereinabove, the circuit element and the repeller electrode are so constructed and arrange that the transit time in the gap to repeller region varies inversely in a manner substantially linear with frequency whereby the optimum electron transit angle in this region is obtained for a wide range of frequencies ithout alteration in the voltage of the repeller electrode.

The above noted and other features of this invention will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is an elevational view mainly in section of a tunable reflex oscillator illustrative of one embodiment of this invention;

Fig. 2 is a fragmentary view, to an enlarged scale, of the gap defining the repeller electrodes in the device illustrated in Fig. 1 and Fig. 3 is a graph showing a typical operating characteristic for a device of the construction illustrated in Fig. l.

Referring now to the drawing, the device illustrated in Fig. 1 comprises an evacuated enclosing vessel it, for example of metal, having a base ll carrying terminal pins 12. Mounted within the vessel I 0 is a cavity resonator l3 bounded in part by a pair of aligned dished grids l4 and 15 which are spaced to define a gap Hi. The grid It is mounted upon the end of a cylindrical metallic member I! extending from a plate 18; the grid is is mounted upon a flexible diaphragm l9 which in turn is supported by a cup-shaped metallic member 29 joined at its edge to the plate It. Energy may be taken from the cavity resonator by way of a coaxial line 2|, 22, the inner conductor 2| of which terminates at its inner end in a coupling loop 23 within the resonator.

Opposite the grid l4 and aligned therewith is an electron gun which comprises a cathode 24 having a concave face 25 at one end coated with electron emissive material, and a heater element 26 within the cylindrical part of the cathode. The electron gun comprises also a cylindrical beam forming or focussing electrode 21 and a cylindrical accelerating anode 2B, which is coaxial with the electrode 21 and fitted within and electrically integral with the reduced end portion of the member ll. The cathode 24., heater element 26 and electrode 2'! are connected to the terminal pins [2 through lead-in conductors 29 hermetically sealed, by vitreous beads 30, in eyelets 3| on the base of the vessel [0.

A dished repeller electrode 32 is mounted opposite and in alignment with the grid [5 by an insulating disc or plate 33 which is mounted rigidly from the resonator member 20 by a support 34.

Tuning of the cavity resonator may be effected by flexing the diaphragm I9. The tuning mechanism includes a spider or bracket 35 the arms of which are connected to a rigid annulus affixed to the diaphragm and the base of which is joined to a stem 36 in turn joined to a diaphragm 3i sealed to the vessel I0. Longitudinal motion of the stem 36 with consequent displacement of the diaphragm I9 and alteration in the natural frequency of the resonator [3 may be accomplished by a nut 38 threaded upon the stem 36 and fixed against axial displacement by a support 40.

In brief, in operation of the device, the resonator I3 is biased positive relative to the cathode 24 and the repeller electrode 32 is biased negative relative to the cathode. A concentrated electron stream is projected by the gun, across the gap I6 Whereat the stream is velocity varied. The velocity varied stream enters the region between the grid I and repeller electrode 32, is reversed in direction because of the potential of this elec trode and injected again in to the gap, thereby to contribute energy to the field within the resonator.

' As has been pointed out heretofore, the operating characteristics of such a device are depend ent upon the total transit time of the electrons in the gap to repeller region. Optimum operation, specifically maximum power output, requires that the transit angle bear a definite relation to the operating frequency. The transit angle depends upon both the gap to repeller electrode distance and the voltage of the repeller electrode.

It is evident that if the diaphragm it is displaced to tune the resonator to a desired frequency, the distance between the gap and the repeller electrode will be varied. In devices of construction known heretofore, this has necessitated adjustment of the voltage of the repeller electrode to obtain maximum power output. The nature of the variation in a typical, well designed prior art device is illustrated by line A in the graph of Fig. 3 wherein the abscissae are operating frequency and the ordinates are repeller electrode voltage requisite for maximum power output.

In accordance with a feature of this invention, such variation is minimized whereby a substantially maximum output is obtained over a wide range of frequencies for a fixed repeller electrode voltage. Specifically, this is realized by correlating the gap to repeller electrode spacing and the frequency so that the transit time in the region between the gap and the repeller varies inversely substantially linearly with frequency over a desired band of frequencies.

The relation involved is given to a high degree of accuracy by the equation where Zis the gap to repeller spacing for the mid-frequency in the desired band,

fl. is the lower frequency of the desired band,

In isdthe upper frequency of the desired band,

5 is the difference between effective gap to repeller spacing for the frequencies, fr. and in.

in Fig. 3. The improvement achieved by this invention is evident from a comparison of graphs A and B. In a specific device constructed in accordance with this invention, over the range of 3800 to 4200 megacycles per second, the maximum variation in repeller Voltage for maximum output was found to be 10 volts (113-103) whereas for a device corresponding to graph A the corresponding variation for the same frequency range was found to be 42 volts (122-80).

Thus, in devices constructed in accordance with this invention, substantially maximum output may be obtained over a band of operating frequencies at a fixed repeller electrode voltage.

What is claimed is:

l. A reflex oscillator comprising a cavity resonator including a pair of juxtaposed members defining a gap, means opposite one of said members for projecting an electron stream across said gap, a repeller electrode opposite the otherof said members, and means connected to said other of said members for displacing said other member to vary the length of said gap thereby to tune said resonator over a preassigned band of frequencies, said repeller electrode being spaced from said gap substantially in accordance with the relation where l is the distance from the center of gap to the repeller electrode at the mid-frequency of said band, f1. and in are the extreme frequencies in said band, and 6 is half the displacement of said one member requisite to tune said resonator over said range.

2. A reflex oscillator comprising a cavity resonator including a pair of juxtaposed electron permeable portions defining a gap, an electron gun opposite one of said portions for projecting an electron stream across said gap, means connected to the other of said portions for moving said other portion toward or away from said one portion thereby to tune said resonator over a preassigned frequency range, and a repeller electrode opposite said other portion and spaced from said gap according to the relation where Z is the distance from center of gap to the repeller electrode at the mid-frequency in said range, fL and in are the extreme frequencies in said range, and 5 is half the displacement of said portion requisite to tune said resonator over said range.

WILLIAM G. SHEPHERD.

REFERENi'JES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

